Burning Rates

Fahlin Racing

Fahlin Racing

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Seeing some interested in combustion, and ENafziger had posted some juicey details. To anyone new to combustion, things will get deep in technical most likely. Everything involved will probably be talked about from chamber surfaces to thermal dynamics etc, I am certainly looking forward to what can unfold in this thread :D

ENafziger wrote:
Higher Cetane diesel does/will/can provide benefit. However, we've tested cetane effects on engine efficiency, and there is no measureable benefit above 55...and hardly any benefit above 52. The only way you can utilize a synthetic 60 cetane fuel to its potential, is to have accurate in-cylinder combustion data and tune things to the ragged edge of combustion stability/pressure rise rate/peak pressure/etc. Then, and only then, will a 60+ cetane fuel benefit you, by minimizing the variation in ignition delay that could be experienced with lower cetane fuels.

Flow enhancers, fuel magnets, aligning fuel molecules, blah blah blah, will not do anything for burn efficiency.

Even a low (40) cetane fuel still has a very high burn efficiency when it lights...it's just prone to more variation on when it lights, sometimes making it more difficult to optimize brake thermal efficiency.
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What engine was your test bed? bore, stroke, crown design etc... single cylinder?
 
Since cylinder pressures are higher in the diesel engine, what do you believe about the cylinder pressures effect on the actual point of ignition and evaporation rate of our atomized fuel Enafziger? Anyone else?
 
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This information doesn't come free. If you know it, you paid the price and it's not cheap. Price out a single in-cylinder pressure transducer and post your results.
 
What engine was your test bed? bore, stroke, crown design etc... single cylinder?
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I'm not at liberty to say. The engine used for that data is a multi-cylinder engine modified to be a single cylinder platform. The block is built by FEV, and the head is an OEM production unit. It is a light duty common rail diesel running Bosch fueling components. Turbocharging is accomplished by using a rotary screw air compressor, refrigerated dryer, and a six-figure mass flow controller and conditioning cabinet.

The data would scale reasonably well to a 5.9/6.7 Cummins. The major difference is that the head design we're using incorporates a swirl vane for control over mixing.

Instrumentation is pretty standard...Kistler pressure transducer, Kistler fuel line pressure, and a BEI crankshaft encoder with 0.2° resolution. The fluid loops (for fuel, oil, air, coolant, EGR, intake temperature, etc) are temperature controlled with PID Honeywell controllers. Data acquisition is a mix of AVL Indimodules, LabView, Drivven's micro DCat, etc. Emissions sampling includes basic 5 gas via California Analytical for intake and exhaust sampling, SMPS, FTIR, AVL Smokemeter, filters for organics, and bag samples for analytical chemistry (GC-MS, etc), wide band lambda sensors, etc.

Eric...
 
Thats been realized, Joe. I was curious on if it was multi-cylinder or single since you can localize your concentration on one area and not have affecting combustion in other areas influencing heat transfer behavior. Thank Eric, the small insight on what you guys get to play with regularly is appreciated. But, back to my other question...

Whats you view on cylinder pressure effect on ignition and evaporation inturn our oxidation rates?

Theres been talk on engines of 14.5 static verse higher static compression engines, perhaps this could be a combination of compression, more area and the amount of swirl employed? hmmmm
 
Fahlin Racing said:
Since cylinder pressures are higher in the diesel engine, what do you believe about the cylinder pressures effect on the actual point of ignition and evaporation rate of our atomized fuel Enafziger? Anyone else?
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Since you made this broad statement I wonder where you are getting this misinformation. Think about how a engine makes HP or for that matter torque. It converts heat energy to mechanical energy by the pressure made in the cylinder. So if you want 500 HP it takes "X" pressure in the cylinder to do it and it really doesn't care if it's gas, diesel or Nitro. What a diesel does do is make more at lower RPM than a gas engine does so in that case only the cylinder pressure has to be higher to make more at the same RPM, but that is only one case of many.

I've measured plenty of super/turbo charged gas/alcohol engines that make the same or better HP at the same RPM as a diesel and in those cases there is more cylinder pressure in those engines than a diesel.
 
Higher peak cylinder pressure or higher average cylinder pressures?
 
kazairl said:
Higher peak cylinder pressure or higher average cylinder pressures?
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This, depends how much pressure is imparted on the piston, when, and how long. I can already see the calculus and differential equations popping up above my head with all these rates flying around.


Mazda's new SkyActive diesel has a 14:1 or 14.5:1 compression ratio to keep NOx emissions down which would seems like a major blow to power but apparently they're able to make it up by injecting earlier in the stroke(+ multiple times) and spreading out the pressurization over a longer period of time.
 
Controlling the average pressure in the cylinder is how ALL diesel manufactures are going to have to go to get the engines to run clean enough for emissions. While its not a bad thing and as a matter of fact it is a better way to produce higher fuel economy and improved torque at lower RPM's it doesn't work as the RPM gets higher. You simply run out of time as engine speed increases to get it done, so you have to start dropping pulses and increasing the amount of fuel in each shot.

While it maybe being done for emissions, it's going to open a few new doors for power as well at lower engine speeds as this technology gets out there and improved parts come out.
 
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Diesel Tech said:
I've measured plenty of super/turbo charged gas/alcohol engines that make the same or better HP at the same RPM as a diesel and in those cases there is more cylinder pressure in those engines than a diesel.
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Actually, This statement is what I was asking about.
 
Typically we work to try and keep the peak pressure down and the average up as much as we can but there are limits that we cannot change in the gas/alcohol engines that I wish we could. Since fuel is injected prior to the intake valve fuel droplets become a real big deal in these applications. When your running WOT you get what you get and the only way to change it is with combustion chamber design changes. No mutli fuel shot in the cylinder like we can with a diesel.
 
I have seen a gasser make close to 500hp with 8.5 static compression. Which is why I have posted what I have.

Efficiency goes up with most and a rise in mechanical compression situations, but some newer diesel have gone the other way with lowering the mech. comp. and still gained efficiency. What does that lead me to, the in-cylinder pressure, fuel droplet evaporation and the rate of heat release. Since pressure keeps things together normally, but its when the heat from the compression stroke is introduced to our fuel charge.

Like Eric posted, the cetane rating of the fuel has different characteristics, not only in ignition quality but point of ignition through our Delay angle of the fuel after injection. The engine with more area in the chamber, 14.5 static, compared to ones with upwards of 20 or so, its this little amount of possiblities that I am jumping into looking at along with the different Cetane rated fuels used, and with what applications.

Would anyone happen to know how many different levels of Cetane are in use?
 
LOTS............... as low as 38 and as high as 60 with everywhere in-between. Most pump fuels seem to run in the 40 - 44 range
 
That many possiblities huh, gives me more to look into. Thank you! For those interested in diesel fuel chemical formulas according to wikipedia, common Diesel fuel has a formula of C12H23 but can range from C10H20 to C15H28.

Found this little piece http://eerc.ra.utk.edu/etcfc/docs/altfueltable.pdf

With the specific gravity I am posting for those who don't know exactly, 1 is the weight of water anything below 1.00 is lighter in weight.
Specific Gravity (SpG) of No. 2 0.81-0.89
Cetane rating 40 - 55
Latent heat of Vaporization Btu/gal @ 60F - 700
Latent heat of Vaporization Btu/lb @ 60F - 100
(check out how low these are compared to the alcohols)

Taylor states in his book, three different levels of diesel fuel. Light, medium and Heavy. in question I am believing we use the light class of diesel fuel for our trucks. Hmmm.
He had also stated the chemical formula, circa 1985 FYI, Light C12H26 with a SpG of .876.

Granted we can't exactly observe diesel combustion as we would in a gasser, because our flame doesn't propagate in the same manner. But since our fuel ignition becomes present whenever our air (oxygen) and fuel become within grasp for our flame to come to life, would it be more beneficial to watch the overall heat release with the actual usable burn angle once our power stroke begins?

As our power strokes out, we have completely uncontrollable energy release, as oxygen is used, our burn becomes slower, this would lead me to believe our dependance on in cylinder mixture quality becomes higher as our burn angle progresses hopefully continuing a usable cylinder pressure.

Would Cetane only influence earlier ignition or could it also assist our burning process as well? hmmm
 
From Wikipedia on how they measure the cetane number
Measuring cetane number

Accurate measurements of the cetane number is rather difficult, as it requires burning the fuel in a rare diesel engine called a Cooperative Fuel Research(CFRTM) engine, under standard test conditions. The operator of the (CFRTM)engine uses a hand-wheel to increase the compression ratio (and therefore the peak pressure within the cylinder) of the engine until the time between fuel injection and ignition is 2.407ms. The resulting cetane number is then calculated by determining which mixture of cetane (hexadecane) and isocetane (2,2,4,4,6,8,8-heptamethylnonane) will result in the same ignition delay
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[ame="http://en.wikipedia.org/wiki/Cetane_number"]Cetane number - Wikipedia, the free encyclopedia[/ame]
 
After reading some from Robert N Brady's Modern Diesel Technology he is stating that our diesel burn rate should be measured by the amount of degrees it takes to get a complete burn. An example he had used was involving a 3406E CAT.

Pg 41 Chapter 1
In this hypothetical example, assume that the start of injection occurs at 20 degrees BTDC and the start of the burn occurs at 15 degrees BTDC. Next comes an uncontrolled burning period that produces the high peak pressures (beyond the normal at TDC) of approximately 3 degrees of crankshaft rotation. The fuel injection is cut off at approximately 1.5 degrees ATDC (after top dead center) and the burn ends at approximately 18 degrees ATDC, for a total burn rate time of about 33 crankshaft degrees. The time required for all of this to happen is extremely short and that few technicians consider when they think about what is going on within an engine during a full-load rpm condition of, say 1,800 rpm.

In this example of burn rate we can determine the times involved by considering the following data. First assume that the piston stroke is that of a 3406E Cat engine, that is 6.5 in., and the engine produces its rated horsepower at 1,800 rpm. According to the piston speed formula, the piston travels at 1,950ft/min (594.3m/min) or 32.5ft/sec (9.9m/s). In one complete turn of the engine crankshaft we rotate through 360 degrees, which is equivalent to the piston stroke times two - 6.5 x 2 = 13in. (330.2mm) - divided by 360; therefore 1 degree = 0.0361in. (0.917mm). The time required to cover each degree is 0.0361 in. divided by 390in., which is 0.0000925 sec. From the start of injection period at 20 degrees BTDC to the start of the burn at 15 degrees, there is a five degree delay period equivalent to 0.0004625 sec. The uncontrolled burning period that creates the very high pressures and temperatures beyond those that normally occur later at TDC lasts for about 3 degrees, or 0.000277 sec. Fuel is injected to about 1.5 degrees ATDC, or 0.000143 sec, with the burn rate ending at approximately 18 degrees ATDC, or for 0.000166 sec ATDC. Therefore, the total burn time lasts for approximately 33 degrees (15 BTDC to 18 degrees ATDC), or for about 0.003 sec.
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Knowing we will have different beginning and ending points for our power stroke, I thought I would post his example since its relevant to the thread. Pretty interesting IMO. Fast burn indeed!

For those that want something to chew on I found this fuel quantity for "x" power level as well...

The amount of fuel that the injection pump or unit injector must deliver to the engine to produce its rated horsepower can be determined by using the following formula:

Q = 454,000 x BHP x BSFC DIVIDED by 60 X NX sp. gr.
= 7567 X BHP X BSFC DIVIDED by N X sp. gr.

Q = quantity of fuel per stroke of the pump or unit injector, measured in cubic millimeters
BHP = Brake Horsepower
BSFC = Brake Specific Fuel Consumption, in lb/BHP/hr
N = pump speed or unit injector speed (rpm) or injections per minute
sp. gr. = Specific Gravity of the Diesel fuel being used
454,000 = volume of 1lb of water, in mm cubed
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:Cheer:
 
Anyone have any thoughts as to how much swirl should be induced for the best pressure rise? Applications (output levels) will most likely differ, however, maybe there is no or very minimal change.
 
Modern Diesel Technology - Robert N Brady page 74
Properties of diesel fuels affecting performance
Cetane number - Ignition Quality measure - affects cold starting, smoke and combustion
Sulfur content - Affects wear, deposits and particulate emissions
Heating Value - Affects power output and fuel economy
Volatility - Affects ease of starting and engine produces smoke
Flash point - Related to volatility and fire hazard in handling
API gravity - Related to heat content, affecting power and economy
Viscosity - Affects injector lubrication and atomization
Cloud and pour point - Affect low-temperature operation
Water and Sedimment - Affect life of filters and injectors
Carbon residue - Measures residue in fuel - can influence combustion
Ash - Measures inorganic residues
Corrosion - Measures possible corrosive attack on metal surfaces

Seeing that viscosity involves proper fuel lubrication qualities or lack there of, brings me back to fuel additives and improving fuel lubricity not only in the lines and pumps but through the injectors as well. In turn, easier to create a good vaporizable atomized charge in the cylinder. Cetane, seems to be more than just a ignition helper but also promotes the burn cycle's behavior in terms of how it all unfolds. Cetane has the apparent ability to smooth the combustion cycle out to a point. Higher the number the smoother our burn becomes.

However our soot still becomes apparent from the flame front being snuffed out by all the oxygen being used or our flame front becoming possibly washed out from excessive fuel?
 
cetane rating of biodiesel is higher. but its energy content is a little less, so that throws a wrench into the comparison. Truck runs smooth on bio. Very quiet idle.
swirl is important for using all the air available and allowing the maximum amount of fuel without smoke. how much is too much? dont think it really matters unless you run so much boost (and therefore intake velocity and swirl) that you pop a head gasket.
There is a minimum requirement though, if you care about how drivable it is on the low end.
I tried some 7 hole injectors in the hopes of getting better atomization but the top end suffered, maybe due to reduced penetration? Went back to 5-hole marine nozzle runs better now, but the change coincided with some other stuff so not really solid data.
 
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